The comparison between international purchasing power and real GDP is very important to the judgment of national power and is the main content of national economic statistics. Although the ICP has experienced more than 50 years, its methodological research should continue. This book gives the research pattern, namely "ICP logic diagram", and puts forward more than 50 methodological issues to be considered. The "pure price ratio assumption" and "equal price ratio assumption" and their impact on the ICP data results are analyzed. It also reviews the important literatures on the recent ICP, especially pointing out that the ICP data results have the measurement risk of "anti-basic facts". This book traces back to "Ryten Report" and explores the principles of spatial economic comparison and the corresponding basic concepts of economics.